E glass, or alkali-free glass, is used to manufacture the most commonly used glass components of continuous glass fiber. The ternary system SiO2—Al2O3—CaO is the basis of the components of alkali-free glass. In a ternary phase diagram, it is calcium feldspar, pseudo-wollastonite, and α-tridymite with a low eutectic point constituted by the liquid phase equilibrium. Its components and weight percentage contents are: SiO2 62%; Al2O3 14.7%; CaO 22.3%.
On this basis, B2O3 is added to the glass composition in place of some of the SiO2, and MgO is added in place of some of the CaO to form the alkali-free glass component that is ordinarily used now. Its typical properties are described in U.S. Pat. No. 2,334,981 and U.S. Pat. No. 2,571,074, and the quaternary system SiO2—Al2O3—CaO—B2O3 is generally its basis. In the glass composition in U.S. Pat. No. 2,571,074, B2O3 with a weight percentage content of 5˜13% is added in place of a portion of the SiO2 in order to decrease the glass molding and liquid line temperatures as well as to aid in the glass melting and the fiberglass drawing and molding. However, addition of a large amount of B2O3 results in high raw material costs for the traditional alkali-free glass and significant environmental pollution.
In the fiberglass industry, the molding temperature refers to the temperature at which the fused glass mass becomes easily drawn and molded. In reality, it is a realm in which the temperature range is equivalent to the temperature when the viscosity is 102.5˜103 cP. In this invention, the molding temperature is the temperature at which the viscosity is 103 cP. The liquid line temperature refers to the temperature at which the crystal nucleus begins to form when the when the fused glass mass cools. In order to avoid any risk of devitrification during the fiberglass drawing process, ΔT value is used to indicate the difference between the molding temperature and the liquid line temperature. It should be accurate and, preferably, greater than 50° C. Greater ΔT values indicate that the fused glass mass has higher stability, which helps the glass fiber drawing and molding.
In addition, the content of all components mentioned in this invention are expressed as “%”, which should be understood as “weight percentage” or “wt %”.
The current mainstream low boron fiberglass composition is basically made up of the SiO2—Al2O3—CaO—MgO quaternary system, of which the MgO content is usually greater than 1%. A boron-free fiberglass composition is described in patent WO96/39362, which is made up primarily of SiO2, Al2O3, CaO, and MgO with little or no costly oxides added, such as TiO2, SrO, MnO, and ZnO. In the preferred regimen, the quantity range of MgO is 2˜3.5%, and this fiberglass composition has a higher ΔT value; however, its molding and liquid line temperatures are relatively high. Excessively high molding and liquid line temperatures will greatly increase energy consumption and accelerate high temperature aging of the kiln and platinum bushing, thereby increasing production costs. A low boron fiberglass composition is described in patent WO01/32576, which is made up primarily of SiO2, Al2O3, CaO, and MgO. In the preferred regimen, the MgO quantity range is 1.5˜4%. The ΔT value for this fiberglass composition is relatively high, and the molding and liquid line temperatures are not high. The SiO2 content, however, is lower (less than 58%), which affects the mechanical strength of the glass to a certain extent.
Fiberglass compositions with MgO content less than 1% are very rare, and there are definite problems with the few patents that are known, specifically that they are unable to meet the requirements of industrialized production. For example, patent WO00/73232 uses a fiberglass composition with less than 1% MgO composition. It is made up primarily of the SiO2—Al2O3—CaO ternary system with a certain amount of B2O3, Li2O, ZnO, MnO, or MnO2 also added in order to decrease the molding and liquid line temperatures. However, it can be seen in its embodiments that either the molding temperature remains somewhat high or the ΔT value is far less than 50° C. Moreover, the cost of the raw materials for this fiberglass composition is very high. Another example is the fiberglass composition described in patent WO03/050049 and used for automotive exhaust systems. By adding less than 1% MgO and more than 1.5% TiO2, the objective of increasing acid resistance and heat resistance is achieved. However, because a large amount of TiO2 is used, this fiberglass composition lacks cost advantage.